724 is the number assigned for counties surrounding Allegheny County in South Western Pennsylvania. With that said, it would make sense to acquire this number for faxes, and voicemail. Both the 412 and 878 numbers are used exclusively for voice calls, and depending on the number dialed will depend on the device that rings. While this page is noted as the Voicemail page, it will also support faxing. This will be done by using a telephony server. More information on the telephony server can be found in another page.
To keep things simple, when someone calls my 724 number, the server will listen for a fax tone. If it detects the fax tone, it will collect the fax, convert it to .pdf, and email it to me. If it does not detect a fax tone, it will send the caller to voicemail to where they can leave a message. This is only possible with the use of the telephony server. Since I am more likely to get voicemail messages, this will be why I would note that this number is voicemail.
On another note, this number will be noted in my contact card as a fax/voicemail number. This will be designed to make choices for the recipient of the card to decide how they wish to contact me. If they wish to ring my phone, they will dial the Cellular number. If they wish to send a fax, or voicemail – then they will dial the Voicemail number.